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The cutout in the subfloor for the toilet flange is too wide, leaving nowhere for me to secure it. The plumber had set some shims underneath the flange but I'm not sure I'd trust those shims to hold up since I'd have to trim them short to fit my flooring around the flange.

The bottom of the flange is raised about 1/2" from the subfloor. I plan on pouring about 3/8" of self leveling compound in this area to bring it up to level with the rest of the floor.

My question is how do I go about securing the toilet flange? I have access from the basement. Could I install some blocking underneath on each side and secure a patch of plywood to that from the top?

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  • Any chance of getting under it to work? I would prefer the flange be supported on four sides, instead of just two ends.
    – crip659
    Feb 21 at 19:02
  • Yes I do have access from the basement underneath.
    – Niko
    Feb 21 at 19:17

1 Answer 1

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I'd be fixing the subfloor. Cut out a rectangle lapping to the center of the adjacent joists, then fit two new pieces with a cut across the center of the hole and tie them together with floated backing. This can all be done from the top side. You want the patch large enough to be robust, but fastening it well to the old subfloor can remedy a smaller patch.

  1. Cut the opening. It'll be easier if you measure and square this well, or you can cut your patches first and trace them for cutting.

  2. Float backing, with construction adhesive and screws, at the edges of the opening for binding new and old subfloor.

  3. Install one of the halves with construction adhesive and screws.

  4. Float scab backing at the joint between the halves, near the toilet, also with glue and screws.

  5. Install the second half, gluing and screwing to the joists and the floated backing.

  6. When you fasten the flange back down, use washers or shims to take up any gap. You don't want to flex it down and stress it. A small height increase won't affect your toilet install.


 TOP (PLAN) VIEW


 |   | <-- joist                                 |   |
 |  _|___________________________________________|_  |
 | | :                                           : | |
 | | :         new subfloor                      : | |
 | | :      .....                                : | |
 | | :     :     : <-- floated backing           : | |
 | | :     :     :                               : | |
 | |-:--------------- (  hole  ) ----------------:-| |
 | | :     :     :                               : | |
 | | :     :.....:                               : | |
 | | :                                           : | |
 | | :                  ......                   : | |
 | | :                 :      :                  : | |
 | | :                 :      :                  : | |
 | |_:___________________________________________|_| |
 |   |                 :      :                  |   |
 |   |                 :......:                  |   |

If you have an odd subfloor thickness and want to do the repair with something like 3/4" plywood or OSB, cut to the joists instead of over them, then glue and screw cleats to the sides of the joists at the right height. Now your new patch is at the right level.

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  • I would add that backing between the existing floor & new subfloor on both pieces (top & bottom in the drawing), and I'd add backing on both sides (left and right) of the hole. I'm sure that was intended, but that's a lot of extra ASCII art work so it was left out.
    – FreeMan
    Feb 21 at 19:30

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